The invention relates generally to measurement of the wear of a wall of a metallurgical vessel. The word "wall" as used here refers to any body comprised of refractory material which is susceptible to be progressively consumed when in contact with fused metal contained in the vessel. The invention may thus be employed for continuously measuring: the wear of a refractory lining, or the wear of organs implanted in a refractory lining (e.g. tuyeres for oxygen injection in a converter), or the wear of elements for injecting agitation gas, or the wear of bottom electrodes in an electric arc furnace.
Consider the example of tuyeres disposed in the lining of a converter, which tuyeres traverse the wall of the converter and serve for injection of a treatment fluid, which may be a gas (e.g. oxygen) or a liquid, into the fused metal, which metal contacts the tuyeres during the treatment. It turns out that the erosion of the refractory material is particularly high in the neighborhood of the zones of injection of the gas (or liquid) into the vessel, which injection occurs at the outlets of the tuyeres. One reason why it is particularly desirable to be able to monitor, as accurately as possible, the advance of such erosion, is that such knowledge enables one to take into account the influence of the erosion pattern itself on the parameters of the injection of the fluid. Another reason is that erosion of the refractory material of and near the tuyeres is difficult to predict, and thus monitoring is important in order to avoid the risk of failure of the lining and the vessel wall or tuyere structure.
Accordingly, there have long been efforts to provide means of measuring the wear of refractory materials by measuring the variation in the thickness of the wall formed by or with said materials.
One early technique consists of implanting in the refractory material of the lining, or of the tuyeres, a plurality of thermocouples at various depths in the direction of propagation of the wear, i.e., generally at various depths in the direction of the thickness of the refractory material. This technique has been used, e.g., for tuyeres comprised of permeable refractory elements of the type known as "LBE," as described in Fr. Pat. 2,518,240. The advance of the erosion is monitored via the signals from the different thermocouples implanted in a given permeable element, wherewith a sharp change in the signal indicates that the metal has arrived in contact with a given thermocouple junction in consequence of erosion of the refractory material surrounding said junction. This method of monitoring wear is necessarily discrete or stepwise, in that a new wear signal does not appear until the wear of the refractory advances by a distance through the thickness thereof which distance equals the separation of two thermocouples in the thickness direction.
More recently, instrument makers have proposed apparatuses which aim to achieve continuous measurement by continuously measuring the electrical resistance of a resistive wire disposed in the refractory material, which wire extends in the direction of the thickness of said refractory material. As the refractory material is eroded, the length of the resistive wire is reduced, along with its electrical resistance. Thus, measuring the resistance enables one to monitor the change in thickness of the refractory material resulting from erosion of the refractory material. In practice, the device employed for continuously measuring the length of an injection tuyere, such as described in Fr. Pat. 2,728,336, is a probe comprised of two electrical conductors extending parallelly and separated by an electrically insulating refractory material. The two conductors are connected to means of measuring electrical resistance capable of measuring the variations in resistance resulting from variations in the length of the conductors as the refractory lining of the metallurgical vessel is eroded, wherewith closure of the circuit is provided by the fused metal itself contained in said vessel. At least one of the two conductors is comprised of a material having sufficient resistivity to enable practicable and accurate measurement. Such probes are used principally for measuring wear at fluid-injection tuyeres, in that, in practice, it is necessary to have substantial movement of fused metal such as occurs near the end of the tuyere in order to disturb the deposit of electrically insulating dross present on the wall, and thereby to ensure good electrical connection between the ends of the two conductors.
According to a preferred embodiment described in the above-cited Fr. Pat. 2,728,336, the probe is in the form of a coaxial cable having a central conductor surrounded by an insulating sleeve comprised of an electrically insulating refractory material, e.g., alumina, which sleeve is in turn surrounded by a metal sheath comprised of, e.g., steel or Inconel.RTM., which sheath serves as the second conductor and provides a strong structure for the probe. The central conductor, comprised of, e.g., a steel-based material or a material based on molybdenum bi-silicide, has a resistance per unit length on the order of, e.g., 45 ohm/m, which is appreciably greater than that of the metal sheath.
The method of fabrication according to the state of the art leads to a probe having an external diameter of c. 1.5 mm; the insulating sleeve comprised of alumina, consisting of tubular elements disposed interiorly of the metal sheath, is quite thick, having a thickness on the order of several tenths of a millimeter. Such probes have low flexibility and are ill suited for implantation into difficultly accessible locations, e.g. tuyeres of converters. Moreover, the distance between the central conductor and the exterior metal sheath is such that there is no assurance that a good electrical link will be provided between them via the fused metal contained in the vessel. Defects in this electrical link result in errors in the measurement of the resistance, and thereby of the erosion, in that fluctuations in the resistance measured do not correspond to changes in the length of the central conductor.
It may even be impossible to employ such a probe where one desires to measure the erosion of a refractory material in a region of the vessel having a continuous refractory lining, because the insulating layer of dross covering the surface of the refractory bricks quasi-permanently interferes with electrical linkage of the two conductors in the probe.